This invention relates generally to the art of dilute phase pneumatic transfer and more particularly to the pneumatic conveying of pulverized solvent refined coal to a burner for a boiler or the like.
Solvent refined coal, also known as SRC-I, is a low sulfur, low ash, solid fuel produced from coal and having a composition such that it could be used as a utility boiler fuel which can be burned under environmentally acceptable conditions. Solvent refined coal is produced by the dissolution and hydrogenation of pulverized coal in a processderived solvent. The resulting process stream is flashed to remove hydrogen and like gases processed via critical solvent, by filtration or by other methods, to remove undissolved coal and ash, and then fractionated to separate byproduct gases and distillable liquids, recycle process solvent, and the solvent refined coal product. The solvent refined coal yield is the hydrocarbon fraction having a boiling point substantially greater than 850.degree. F. and generally represents 40 to 70 percent of the moisture ash free feed coal.
Testing is underway by the U.S. Department of Energy to determine the suitability of solvent refined coal as a utility boiler fuel. It would be desirable if the solvent refined coal could be used in existing coal-fired units with a minimum of retrofitting and capital cost.
As to its physical characteristics, solvent refined coal typically has a sintering temperature of 170.degree. F., a melting point of 284.degree. F., a specific gravity of 1.24 grams per cubic centimeter and a bulk density of 50 lbs. per cubic foot as received. Also, solvent refined coal has a heating value which is considerably higher than that of coal, namely, about 15,600 Btu/lb under dry conditions. Also, solvent refined coal has a low melting point, in the range of 280.degree.-300.degree. F., and can become tacky at temperatures as low as 170.degree. F. Thus, for successful pulverization, the internal mill temperature should not exceed approximately 150.degree. F. In addition, to avoid fouling and coking of solvent refined coal on burner surfaces, cooling of the burner is recommended.
Furthermore, solvent refined coal is very friable and has a Hardgrove Value typically greater than 170 as compared to coal which normally never exceeds 100.
It has been found that, as compared to coal, the pulverizing of solvent refined coal results in the mill fineness increasing 20 to 30% in the fraction passing 200 mesh, in conjunction with a 25% drop in the mill power consumption.
In the initial testing by the U.S. Department of Energy of the firing of pulverized solvent refined coal in utility boilers, the testing equipment was designed to transport the solvent refined coal to the burner pneumatically using ambient air as the transport fluid. The assumption of the initial design was that the design parameter values for pneumatic transport systems conveying coal could be used for the designing of the transport system for the pulverized solvent refined coal. However, while this system was conservatively designed based on transporting of pulverized coal, the system exhibited unstable operation when transporting pulverized solvent refined coal.
This type of system for the transporting of pulverized solvent refined coal to a burner, as in the above-discussed testing apparatus, pertains to the well-known art of dilute phase pneumatic transfer. The principles of this art pertaining to the handling of pulverized coal and the like are set forth in the following references:
1. Frederik A. Zenz and Donald F. Othmer, "Fluidization and Fluid-Particle Systems". Library of Congress Catalog Card No. 60-10505. PA0 2. Technical Paper, "Babcock & Wilcox's experience with two-phase flow mixtures of coal and gas", presented by A. G. Bolumen to Conference on Coal Feeding Systems, California Institute of Technology, Pasadena, Calif.--June 21-23, 1977. PA0 3. "Pulverized-Coal Transport through Pipes", by R. D. Patterson, JOURNAL OF ENGINEERING FOR POWER, Jan., 1959. PA0 4. STEAM, Pages 9--9 thru 9--11, "Preparation and Utilization of Pulverized Coal", published by Babcock & Wilcox Company.
It was believed that the above problem of unstable operation was due to saltation (choking) of the solvent refined coal in the transport line and subsequent re-entrainment thereof. Saltation causes unsteady boiler operation, which is inefficient and hazardous. For dilute phase transport there is a minimum velocity below which the solids begin to settle in horizontal flow, this velocity being known on this art as the "saltation velocity".